It is known to produce toroidal sealing rings, in particular as flange packings, from expanded polytetrafluoroethylene (ePTFE). By way of example, uniaxially oriented, expanded polytetrafluoroethylene is used for the production of seals of this kind. However, known toroidal sealing rings of this kind still leave a considerable amount to be desired in terms of the transverse strength, the creep and the consolidation behavior. Furthermore seals of this kind require closure areas, for example, by means of overlapping. In the case of ePTFE seals which have been oriented only uniaxially, considerable densification of the material occurs owing to transverse flow, as does a considerable degree of creep in the transverse direction.
U.S. Pat. No. 5,281,475 has disclosed a continuous PTFE fibre, for example in the form of a strand, a thread, a rod or a tube, this fibre having a helically rolled self-adhesive, continuous film of PTFE. Furthermore, this fibre has an outer surface which has an essentially round profile and a single, spirally running seam. The continuous PTFE film may be filled with particulate fillers or may be coated with polymeric materials prior to production of the continuous, essentially round PTFE fibre. The continuous PTFE layer may be provided for combining with other elements, for example thread-like elements or film-like elements within the continuous, essentially round PTFE fibre.
"Helically rolled" is in this context understood to mean that the resulting PTFE fibre acquires a spiral structure which is formed by winding the continuous PTFE film around a central longitudinal axis of the fibre in the longitudinal direction of the fibre. The PTFE film used here is in particular a film made of expanded, microporous polytetrafluoroethylene.
A similar PTFE fibre is known from U.S. Pat. No. 5,364,699, it being possible for this like elements, in addition to the layers of the helically rolled, self-adhesive, continuous PTFE film. Thread-like elements of this kind may be, for example, conductive wires, optical fibres or fine tubes.
U.S. patent application Ser. No. 08/050,903, with application date Apr. 20, 1993, furthermore describes a composite sealing material which has a core made of elongate polytetrafluoroethylene and at least one film made of a porous, expanded polytetrafluoroethylene, this film being wound, for example helically, around the core.
Furthermore, U.S. patent application Ser. No. 08/300,018, with application date of Sep. 2, 1994, describes a sealing element which is made from a film of expanded polytetrafluoroethylene which is wound helically to form a packing cord and has a relatively low creep.
Porous, expanded polytetrafluoroethylene (ePTFE) is generally known from U.S. Pat. No. 3,953,566. Porous, expanded polytetrafluoroethylene has a higher strength than the unexpanded form of PTFE, is as chemically inert as conventional PTFE and, when it is used as a sealing element, can be employed in a wide temperature range of up to about 350.degree. C. An example of a sealing material made of porous, expanded polytetrafluoroethylene is GORE-TEX.RTM. Joint Sealant, which is produced by W. L. Gore & Associates, Inc., Elkton, Md., USA.
As a consequence of dimensional changes owing to creep, many commercially available PTFE seals require an additional clamping force, which has to be applied to the seal some time after the seal has been installed. This represents an extraordinarily undesirable property for a sealing material. For applications in which it is necessary to maintain a specific height of the seal during use, for functional and safety reasons, it is not possible repeatedly to adjust the clamping force. If such seals are used, for example, as static sealing elements between multiple plates of heat exchangers and frame filters, there is a combined reduction in thickness of the sealing material between the plates owing to creep and, if this is multiplied by the number of plates in the unit, this may result in a considerable reduction in the sealing clamping force, which may lead to substantial leakage problems which cannot be eliminated.
W. L. Gore & Associates, Inc., USA produces a large number of sealing materials which have a core made of a porous, expanded polytetrafluoroethylene which is wound into a strip of expanded polytetrafluoroethylene. A commercially available example of sealing material of this kind is GORE-TEX.RTM. sealing material for use in sealing plate heat exchangers. The advantage of a two-stage design of this kind consists in the fact that the winding of a strip of higher strength around the core material tends to counteract the flow of the core material out of its original dimensions (i.e. the wound strip limits transverse expansion of the core under load). Although this material works reasonably well, there nevertheless remains a need for an improved sealing material made of expanded PTFE which withstands creep. FIG. 8 shows, by way of example, a conventional sealing element 30 made of expanded polytetrafluoroethylene (ePTFE). Owing to the tendency of the material of this sealing element 30 to creep when it is subjected to a compression force in the Z-direction over time, the material tends to expand in the Y direction and to a small extent also in the X direction, as a result of which the seal provided by this material is impaired. The resistance to creep is thus provided primarily in the longitudinal or X direction, but optimum functioning of the seal requires there to be resistance to creep in both the longitudinal or X direction and in the transverse or Y direction.
Furthermore, FIG. 23 shows an exemplary embodiment of a packing cord 22, as described in the above-mentioned U.S. patent application Ser. No. 08/300,018 with application date of Sep. 2, 1994, a packing cord 22 of this kind being used as flange packing between two flanges 20 and 21. In this application, the required closure area formed by overlapping has likewise proven disadvantageous in the case of the packing cord.
DE-C 111 35 56 has disclosed an apparatus for producing wound annular bodies from material which is resilient to a certain degree, such as preferably woven fabrics or knitted fabrics which are impregnated with rubber, synthetic resin or the like, films, sheets or layers of rubber, paper or metal, this apparatus having a mandrel over which a reverse-drawing hose is arranged, onto which the material to be wound is applied and, for the purpose of winding the material, the hose is taken off the mandrel by turning it back on itself. In this known apparatus, the reverse-drawing hose which has been pulled onto the mandrel may, for example, be a section of a flexible tube made of rubber or another sufficiently flexible or elastic substance with or without inserts, or alternatively this reverse-drawing hose may also be formed from a plurality of individual strips. The annular bodies which can be produced by means of this known apparatus may, for example, be used as V-belt blanks or as similar endless annular bodies.